1. Filed of the Invention
The present invention relates to a sheet-feeding mechanism used in image forming apparatuses such as a copying machine, a printer, and a facsimile machine. The present invention relates also to a sheet-feeding device and an image forming apparatus provided with the sheet-feeding mechanism.
2. Description of the Related Art
In conventional image forming apparatuses such as a copying machine and a printer, a sheet-feeding device is provided for feeding sheets which are subjected to image forming. FIG. 17 schematically shows a configuration of such image forming apparatus.
As shown in FIG. 17, an image forming apparatus 100 includes an image forming section 130 and an image fixing section 140. The image forming section 130 is adapted to create an image in a form of a toner image and causes the toner image to be formed on a sheet which is fed from a manual feeding tray 110 or a sheet-feeding device 120. The image fixing section 140 fixes the toner image formed on the sheet to provide a printed object.
Further, under the image forming section 130, there are provided a plurality of sheet-feeding devices 120 in a stacked state.
Each sheet-feeding device 120 includes a tray 125 for holding sheets, and a sheet-feeding mechanism 200, which is indicated by surrounding dotted lines in FIG. 17, for guiding the sheets from the tray 125 to the image forming section 130. The sheets conveyed from the tray 125 and the sheet-feeding mechanism 200 are conveyed to the image forming section 130 through a common conveying passage.
Next, FIG. 18 is a side view schematically showing relevant parts of the sheet-feeding device 120. As shown in FIG. 18, in the sheet-feeding device 120, the tray 125 is fixed to a housing bottom surface 126 of the sheet-feeding device 120 rotatably through a rotating mechanism 127, and held in a state of inclining at a predetermined angle.
Further, the pick-up roller 211 is accommodated in a housing 210, which is made of resin, together with a feeding roller 212 which will be described herebelow. The housing 210 is mounted on a housing upper surface, which is not illustrated, of the sheet-feeding device 120 through an elastic member 213, and rotated about a rotational axis of the feeding roller 212 downward in the drawing by a pressure exerted by the elastic member 213. This causes the pick-up roller 211 to come in contact with a surface of sheets held on the inclining tray 125. FIG. 18 shows a state where the sheets are not placed on the tray 125, and the surface of the tray 125 is in contact with the pick-up roller 211 directly.
Next, the retard roller 215 is provided on a side of the housing of the sheet-feeding device 120 so as to face the feeding roller 212 provided in the housing 210. The elastic member 217 exerts a biasing force to cause the retard roller 215 to move toward the surface of the feeding roller 212, so that the retard roller 215 is pressed against the feeding roller 212 at a predetermined nip pressure.
The feeding roller 212 is a roller which is constantly rotated by an unillustrated external drive source and causes the pick-up roller 211 to be rotated in a rotational direction which is the same as the rotational direction of the feeding roller 212, through an intermediate gear 214.
Further, the retard roller 215 normally rotates in the opposite direction with respect to the rotational direction of the feeding roller 212, and it rotates in a forward direction in accordance with a load exerted to an unillustrated torque limiter. Further, before a nip portion N formed by the feeding roller 212 and the retard roller 215, a part of a housing wall surface of the sheet-feeding device 120 is provided as a cover 216.
FIG. 19 is a perspective view schematically showing a configuration near the retard roller 215 of the sheet-feeding device 120. Further, FIG. 20 is a perspective view schematically showing a positional relationship between the feeding roller 212 and the retard roller 215 in the sheet-feeding device 120.
As shown in FIG. 20, the cover 216 serves as a guide plate for guiding a sheet conveyed from the tray 125 to a position between the feeding roller 212 and the retard roller 215 along with a plurality of fins 220 provided at predetermined intervals on opposite sides of the retard roller 215. As shown in FIG. 18, the sheet is guided along the cover 216 to the nip portion N formed between the feeding roller 212 and the retard roller 215, and then is conveyed to the image forming section 130.
The feeding roller 212 and the retard roller 215 are mechanisms for preventing multi-feeding of the sheets. In other words, as shown in FIG. 21, the sheet sent out by the pick-up roller 211 to the feeding roller 212 is not necessarily one sheet at each time. There is a case where a plurality of sheets are sent out at one time in a stacked state due to a friction between surfaces of the sheets.
For example, when two stacked sheets 300a and 300b are conveyed to the nip portion N formed between the feeding roller 212 and the retard roller 215, the upper sheet 300a comes in contact with the surface of the feeding roller 212 rotating in a conveying direction and is sent to inside. On the other hand, rotation of the retard roller 215 in the opposite direction with respect to the rotational direction of the feeding roller 212 pushes back the lower sheet 300b toward the opposite direction with respect to the conveying direction of the sheet 300a indicated by an arrow in FIG. 21.
When only the sheet 300a is fed by the pick-up roller 211, the feeding roller 212 and the retard roller 215 exert a force to the sheet in opposite directions. However, when a load which is greater than a predetermined amount of load is applied to the torque limiter, the retard roller 215 is rotated in the opposite direction, in other words, a direction of following the rotational direction of the feeding roller 212. Thus, the sheet 300a is sent toward the conveying direction of the arrow shown in FIG. 21 by respective rotations of the feeding roller 212 and the retard roller 215.
In the operation of the sheet-feeding mechanism 200, it is required that the feeding roller 212 assuredly separates the upper sheet 300a and the sheet 300b. In such technology as disclosed in Japanese Patent Unexamined Publication H11-59937 (hereinafter, referred to as “patent document 1”), as shown in FIG. 22A and FIG. 22B which is a sectional view taken along the A-A line of FIG. 22A, there is a form in which a pair of belts 400 are used in place of a feeding roller. This form is so configured that a retard roller 450, which has a projecting portion 452 having an outer diameter larger than that of a main body 451, is provided at a position corresponding to a mid portion of the belt 400, and the projecting portion 452 of the retard roller 450 fits into a clearance between the pair of belts 400 to form a nip portion N.
In the retard roller 450, fine irregularity is formed on the surface of the projecting portion 452 so that the friction coefficient of the surface of the projecting portion 452 is greater than that of the surface of the main body 451.
According to such configuration, the sheet to be conveyed is sandwiched between the belts 400 and the main body 451 of the retard roller 450, and the sheet to be pushed back comes in contact with the projecting portion 452 at the clearance between the belts 400, so that separability to separate the sheet to be conveyed to the image forming section 130 from other sheet is improved.
However, in the conventional sheet-feeding mechanism shown in FIGS. 22A and 22B, the following problem is present. In particular, since the projecting portion 452 of the retard roller 450 has an outer circumference which is larger than other parts, as schematically shown in FIG. 23, an end portion of the sheet, which is to be conveyed to the nip portion N in the conveying direction indicated by the broken line in FIG. 23, certainly comes in contact with the projecting portion 452 firstly regardless of the number or kind of the sheet.
The projecting portion 452 as a part of the retard roller 450 is rotated in the opposite direction with respect to the conveying direction of the sheet, and the surface is in a rough state having irregularity. Therefore, it was likely that, when the sheet to be conveyed collides with the projecting portion 452, the end portion and the surface is damaged or deformed.
Further, the sheet is sandwiched between the belt 400 and the retard roller 450. The surface of the projecting portion 452 of the retard roller 450 is rough due to irregularity formed thereon and is rotated in such a state of being fitted between the belts 400. It was likely that the surface of the sheet is damaged when it comes in contact with the surface of the projecting portion 452, and as shown in FIG. 24, it was likely that a trace of contact with the projecting portion 452 appears on a sheet 500 as a belt-like scratch 501. Especially when only one sheet is fed by the pick-up roller 211, the surface of the sheet which is conveyed to the conveying passage and subjected to image forming is likely to be damaged.
The present invention was made in view of the problems described above, and its object is to provide a sheet-feeding mechanism capable of improving separability with respect to other sheet without damaging sheets, and a sheet-feeding device and an image forming apparatus provided with the sheet-feeding mechanism.